Reverse engineering fine-tunes NASCAR engine

Several race teams rely on the Standard General Motors engine block, the SB2, to power their NASCAR racers.

Engineers at Richard Childress Racing digitized a standard General Motors engine block, the SB2, giving NASCAR teams that use it a starting point for future redesigns and modifications.

Bob CramblittContributing editor

It's called stock-car racing, but the cars are anything but stock. By the time NASCAR racers hit the tracks, only the outline of the car bodies resemble anything found on the showroom floor.

Some of the biggest changes are reserved for the engine. For teams such as Richard Childress Racing -- home of Winston Cup drivers Kevin Harvick and Robby Gordon -- the challenge is to squeeze the most horsepower and torque possible out of an SB2 General Motors engine and stay within NASCAR rules.

The original small-block V8 was introduced in 1955. Over the years, modifications on the original design have increased durability and accommodated new cylinder-head designs. But with an original design dating back nearly half a century, there was no 3D digital model of the block, only 2D prints.

Last year, GM's racing division asked Richard Childress Racing to create the first 3D digital model of the SB2 engine block. A logical starting point for traditional reverse-engineering projects would be the 2D drawings. In this case, however, 2D prints don't hold the whole story. When 3D molds and cores are constructed, changes are made that are not necessarily in the drawings. RCR wanted to capture these details as they appear in the finished part.

Complicating matters further, the engine shapes are difficult to reproduce using direct CAD geometry, even after precise measurement. "The geometry of the casting -- the shape, draft, water jackets, and internal cores -- is complex," admits Clifton Kiziah, the senior engineer who headed the project. Radius rounds from an actual casting, for example, might be spline-shaped, not spherical with a single radius as depicted in most CAD models.

"And we wanted a model of what we have, rather than an idealization of what we think we have," says Kiziah. "So the 3D model needed to match the physical block as closely as possible."

The 3D model also had to be accurate, meeting casting tolerances of ±0.030 in. Anything less would compromise the credibility of FEA and CFD testing results.

Geomagic software let RCR engineers capture data from an actual SB2 block and convert it to a 3D surface model. Kiziah took urethane molds of the internal cavities and sent them with an entire as-cast engine block to ADC, a Milwaukee-based scanning and reverse-engineering company.

ADC used an ATOS white-light scanner to capture cores and the engine block as eight separate point-cloud files. The engine block was a 20-Mbyte file and cores consumed about 15 Mbyte each.

After receiving the point-cloud files, Kiziah did a uniform sampling operation to thin out the number of points. He then created a polygon model and repaired data holes to make the model "airtight." The polygon model was converted to Nurbs surfaces and fine-tuned by Kiziah.

The eight surface files, complete with datum planes, axes, and references created in Geomagic Studio, were imported into Pro/E. "Critical datums showed up in Pro/E, where they were used as the foundation for the machined features on the block model," says Kiziah. "In effect, we were simulating in software how a block is physically machined."

Kiziah's final model had three times the accuracy required by casting, tightening the tolerance to 0.010 of an inch, and in some cases to within 0.005.

"It would probably have taken several months if we had to model the block directly, and I'm not sure we would have been able to capture the complexity of the actual cast surfaces," says Kiziah. He eventually exported the final 90-Mbyte model as an IGES surface so it could be imported into GM's Catia software.

GM and its racing teams now have an accurate digital model of the SB2 engine block for engineering analysis. RCR ran CFD tests on the model to optimize cooling, as well as secondary machining simulations to check for clearances and fit of new parts. FEA simulations will determine where material can be removed in secondary machining without affecting block strength. The RCR team is also working on a digital model of a total car assembly, complete with the chassis and the entire engine.

SB2, Foundation for Horsepower

The Small Block, Second-Generation is the first engine designed by General Motors specifically for NASCAR racing. Having an "off-the-shelf" engine simplifies preparation for racing teams and reduces the overall cost of building and maintaining a cutting-edge racing engine.

The engine was presented to NASCAR officials in late 1995, and won approval for Winston Cup competition just prior to the 1998 racing season. It is the basic engine powering some of NASCAR's legendary drivers, including the late Dale Earnhardt, a member of the RCR team for 20 years, and Jeff Gordon.

For Winston Cup and Busch Series race teams, the SB2 is just the starting point for building competitive engines. NASCAR teams spend hundreds of thousands of dollars and tap into a treasure-trove of skill and imagination to get 800 hp out of it for Winston Cup racing and 700 hp for the Busch Series. The carburetors in SB2 engines deliver more than 800 cfm of air/fuel mixture and get about 4 mpg. In comparison, the original small-block engine produced 162 hp.